No it's about control. The laws cost more to enforce than they get in revenue.
The laws on both silencers and short rifles were "sold" on the basis that only criminals would need to conceal the sound of their shooting, with an implication of assassinations, and the same is true of short rifles, only criminal assassins would "need" them.
But of course the NFA is just as unconstitutional as any other gun control law that bans whole classes of arms from general ownership, or taxes such ownership. A tax on exercising a Constitutionally protected right, unless it's part of a broader tax, like a general sales tax, is an "infringement".